An electric traction vehicle is a vehicle that uses electricity in some form or another to provide all or part of the propulsion of the vehicle. This electricity can come from a variety of sources, such as stored energy devices relying on chemical conversions (batteries) to create electrical energy, stored energy devices relying on stored electrical charge (capacitors), stored energy devices relying on mechanical stored energy (e.g., flywheels, pressure accumulators), and energy conversion products. In a typical conventional electric traction vehicle, a prime mover, such as a diesel engine, is used to drive an electric generator or alternator which supplies electric current to one or more traction motors. The traction motors typically are coupled to wheel sets on the vehicle. A typical vehicle that utilizes this type of electric traction is a railroad locomotive. In some conventional electric traction vehicles, stored energy is used to provide the main power which provides the electrical current to one or a plurality of traction motors. A typical vehicle that utilizes this type of electric traction is a golf cart or battery powered electric car. In some conventional electric traction vehicles, having more than one source of energy is desirable, such as a stored energy unit and an internal combustion engine coupled to a generator. By having more than one source of energy, some optimizations in the design can allow for more efficient power production, thus allowing power to be used from different sources to come up with a more efficient system for traction. These types of vehicles are commonly referred to as hybrid electric vehicles (HEV). Series and Parallel HEV system designs are what is usually encountered.
In a typical electric traction system of an electric traction vehicle, an electronic vehicle controller may be used to control the torque output of the electric traction motors. The vehicle controller may also be configured to reduce or prevent wheel slip or wheel locking. Wheel slip occurs during motoring when the amount of torque output from the motor produces a force greater than can be applied to a surface in contact with a wheel coupled to the motor, causing an increase in the speed of the wheel with respect to the speed of the electric traction vehicle. Wheel locking occurs during regenerative braking when the amount of torque output from the motor produces a force greater than can be applied to a surface in contact with a wheel coupled to the motor, causing a decrease in the speed of the wheel with respect to the speed of the electric traction vehicle. In a typical electric traction system of an electric traction vehicle, an electronic vehicle controller may attempt to prevent wheel spin or wheel locking by monitoring the speed of each wheel coupled to each motor, and to adjust the torque reference value for a particular motor using the speed of the wheel coupled to the motor so that the amount of torque output from the motor does not produce a force greater than can be applied a surface in contact with the wheel coupled to the motor.
Processing wheel speed feedback signals in the vehicle controller increases the overall amount of processing required in the vehicle controller, and can lead to time delays associated with transmitting and processing the various wheel speed feedback signals and the torque reference value. If significant delays occur during the process of obtaining the feedback signals, processing a change in the torque reference value, and transmitting the adjusted torque reference value, the vehicle may become unstable. Thus there is need for a system and method for reducing wheel slip and wheel locking in an electric vehicle which uses a motor drive controller rather than the vehicle controller to reduce wheel slip and wheel locking, and which controls wheel slip and wheel locking without adjusting the torque reference provided by the vehicle controller.